The following documents concerning the stability of compounds which contain arsenic are available. Patent document 1 presents a method of producing scorodite from arsenic contained in smelting smoke and ash.
Patent document 2 presents a method of leaching arsenic sulfide where air is blown into a slurry containing arsenic sulfide while adding an alkali, in order to leach out arsenic while maintaining the pH between 5 and 8.
Non-patent document 1 reports on the solubility product of iron arsenate, calcium arsenate, and magnesium arsenate. According to this document, calcium arsenate and magnesium arsenate are stable only in the alkali region, but iron arsenate is stable from the neutral to acidic region, and the minimal solubility at a pH of 3.2 was reported to be 20 mg/l.
Non-patent document 2 discloses the solubility of iron arsenate and scorodite. This document shows that the solubility of arsenic from scorodite in the weakly acidic region is two orders of magnitude smaller than that of noncrystalline iron arsenate, and discloses that scorodite is a stable arsenic compound.
Non-patent document 3 presents a method of producing scorodite from arsenic contained in sulfuric acid plant waste water and smelter waste water.    Patent document 1: Japanese Patent Application Laid-open No. 2005-161123    Patent document 2: Japanese Patent Publication No. S61-24329    Non-patent document 1: Tadahisa Nishimura and Kazumitsu Tozawa, Res. Inst. of Mineral Dressing and Metallurgy, Tohoku University, No. 764, Vol. 34, Edition 1, Reprint June 1978.    Non-patent document 2: E. Krause and V. A. Ettel, “Solubilities and Stabilities of Ferric Arsenate Compounds” Hydrometallurgy, 22, 311-337, (1989)    Non-patent document 3: Dimitrios Filippou and George P. Demopoulos, “Arsenic Immobilization by Controlled Scorodite Precipitation” JOM Dec., 52-55, (1997)